Poor type error messages (refer to generated code rather than the higher level problem)
With this module:
module Foo where
data Foo a = Foo
data Bar a = Bar (Foo a)
deriving Eq
the HEAD says:
q.hs:5:0:
Non type-variable argument in the constraint: Eq (Foo a)
(Use -fglasgow-exts to permit this)
In the context: (Eq (Foo a))
While checking the context of an instance declaration
In the derived instance: (Eq (Foo a)) => Eq (Bar a)
which isn't really the error message that we want, as it's refering to the generated instance.
- 6 gives the much better:
q.hs:5:5:
No instance for (Eq (Foo a))
arising from the 'deriving' clause of a data type declaration
at q.hs:5:5
Possible fix: add an instance declaration for (Eq (Foo a))
When deriving the instance for `Eq (Bar a)'
This example is from tcfail046; tcfail169 is similar and tcfail118 is also a related issue.
Trac metadata
Trac field | Value |
---|---|
Version | 6.6.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | high |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | Unknown |
Architecture | Unknown |